Certain ultrasonic transducers which may be used to radiate ultrasonic waves into a fluid medium and/or to absorb ultrasonic waves from the fluid medium are conventional. For example, ultrasonic transducers of this type may be used in ultrasonic flowmeters, for example in process engineering and/or in the automotive industry. Examples of ultrasonic transducers are described in German Published Patent Application No. 10 2007 010 500, German Published Patent Application No. 42 30 773, and European Published Patent Application No. 0 766 071.
Certain conventional ultrasonic flowmeters have air ultrasonic transducers based on a piezoceramic and which include an impedance matching layer, for example a λ/4 impedance matching layer. The impedance matching layer is used to bridge the great difference in acoustic impedances between air and ceramic. As a rule, the material of the impedance matching layer must have, on the one hand, a damping effect which is not too excessive and, on the other hand, an acoustic impedance which lies between the impedance of the fluid medium and the impedance of the piezoceramic. The theoretical optimum value for plane waves usually lies in the geometric mean of these two impedances. Due to the low air impedance, this theoretical value may usually be achieved only with the aid of extremely porous materials, such as aerogels, which, however, are less sturdy. In reality, useable results may also be obtained with the aid of high-density syntactic foams. A frequently used material for the impedance matching layer is epoxy resin filled with hollow glass spheres. Porous sintered polyimide may also be used. In many cases, a sequence of multiple layers is also used instead of a single matching layer, so that, for example, a higher ultrasonic amplitude or ultrasonic bandwidth may be achieved using step-by-step impedance matching. An additional layer may also be used to protect the piezoelectric transducer element against thermally induced tensile or shearing stresses.
Due to the impedance matching actually achieved between the piezoelectric member and the air, it is usually difficult to inject a sufficient amount of sound energy into the air or to receive it from the air in real air ultrasonic transducers, as opposed to applications in liquids. With the exception of an extremely small fraction of the actual useful signal, a large portion of the sound energy is usually reflected back to the limiting surfaces of the piezoelectric member or of the impedance matching layer. On the other hand, the injection of the structure-borne noise from the piezoelectric member into its attachment, for example via a damping casting, a decoupling element, a transducer housing, or the like, is usually counteracted by only comparatively small impedance differences. Even if a transducer core is embedded into a soft silicone, a parasitic sound transfer to the surrounding materials and/or to a second transducer usually still occurs, which is implemented in many cases, for example, to measure flow. This parasitic sound transfer is usually much stronger that the actual useful signal through the air, and it also usually does not sufficiently subside until the actual useful signal arrives. Accordingly, a structure-borne noise decoupling is used in many cases by implementing silicone- and/or elastomer-molded parts, such as O rings. Decoupling is usually improved by geometric shaping, for example by reducing the geometry to a merely linear coupling of an O ring to the transducer core, if necessary also in vibration nodes of the transducer housing.
An ultrasonic transducer for use in a fluid medium is described in German Published Patent Application No. 10 2008 055 126. In this ultrasonic transducer, it is described, among other things, to provide a decoupling element between a housing of the ultrasonic transducer and a matching body. Elastomer materials, which may include gas-filled hollow spheres or gas inclusions, are proposed as examples of materials of decoupling elements of this type.
A disadvantage of these approaches, however, is a usually insufficient media resistance of the decoupling materials used or of the connecting points between the decoupling material and the adjacent materials. This disadvantage may usually be overcome by a suitable coating, for example a Teflon coating, a parylene layer, or a lacquer layer. In principle, a film may also be used for sealing, for example in connection with an open-pore matching layer, made for example of porous sintered polyimide. If the ultrasonic transducer is also to withstand pressure loads, the coating and/or the film should, however, be in direct and continuous contact with the impedance matching layer, the decoupling material and the further transducer environment, for example a sleeve or a housing, to be able to withstand corresponding forces. This is extremely difficult, in light of assembly tolerances of the relatively soft decoupling molded parts.